The invention relates to a convergence adjustment device for video-projectors comprising several cathode ray tubes.
A video-projector is a television receiver projecting images on a larger sized screen than the usual screens of cathode ray tubes. The most widely used type comprises three cathode ray tubes, one for each fundamental color and, for each of these tubes, a lens for projecting onto the screen. The images produced by the three tubes must be perfectly superimposed on the screen. This result is practically impossible to attain by simple adjustments of the orientation of the projection tubes and lenses. The reasons for this impossibility are diverse: the dispersion, inherent in large scale manufacture, of the forms of the images produced by each of the tubes; the lenses, usually made from a plastic material which, for reasons of economy, are not provided with chromatic corrections and which therefore do not have the same index of refraction for the three fundamental colors; not all of the axes of the three tubes may be perpendicular to the projection screen; in general, the axis of the tube projecting the green image is theoretically perpendicular to the screen and the axes of the tubes projecting the red and blue images are slanted in opposite directions with respect to this perpendicular; thus, the green image may be rectangular whereas the red and blue images have the form of a trapezoid with vertical parallel edges.
This is why a video-projector comprises a convergence correction or adjustment device which generates currents feeding coils acting on the horizontal and vertical deflections of the electron beam of two of the tubes, in general the red and the blue, so as to shape the corresponding images so that they are superimposed on the screen on the image projected by the first tube, namely the green. This correction is effected either directly on the line deflection (horizontal) and frame deflection (frame) coils using active elements and modulators or by means of auxiliary deflectors.
Up to now, in order to provide such convergence adjustment during installation of the video-projector, analog circuits are used having potentiometers which are adjusted so that convergence correction signals are formed which are possibly variable from one zone to another of the image.
The circuit of the invention is of the digital type, and so inexpensive while nevertheless providing a great number of adjustments carried out by persons having no particular competence.